In a CCD type solid-state imaging device or a CMOS type solid-state imaging device mounted in a digital camera, a large number of photoelectric conversion devices (photodiodes) serving as photo acceptance portions and signal readout circuits for reading out photoelectric conversion signals obtained by the photoelectric conversion devices to the outside are formed on a surface of a semiconductor substrate. In the CCD type solid-state imaging device, each of the signal readout circuits includes a charge transfer circuit, and a transfer electrode. In the CMOS type solid-state imaging device, each of the signal readout circuits includes an MOS circuit, and a signal wiring.
Accordingly, in the solid-state imaging device according to the related art, both the large number of photo acceptance portions and the signal readout circuits have to be formed together on the surface of the semiconductor substrate. There is a problem that the total area of the photo acceptance portions cannot be enlarged.
In addition, in a single plate type solid-state imaging device according to the related art, one of color filters, for example, of red (R), green (G) and blue (B) is stacked on each photo acceptance portion so that each photo acceptance portion can detect an optical signal with corresponding one of the colors. For this reason, for example, a blue optical signal and a green optical signal in a position of a photo acceptance portion for detecting red light are obtained by applying an interpolation operation on detection signals of surrounding photo acceptance portions for detecting blue light and green light. This causes false colors to thereby result in lowering of resolution. In addition, blue and green light beams incident on a photo acceptance portion covered with a red color filter are absorbed as heat to the color filter without giving any contribution to photoelectric conversion. For this reason, there is also another problem that light utilization efficiency deteriorates and sensitivity is lowered.
While the solid-state imaging device according to the related art has various problems as described above, development on increase in the number of pixels has advanced. At present, a large number of photo acceptance portions (e.g. equivalent to several million pixels) are integrated on one chip of a semiconductor substrate, so that the size of an aperture of each photo acceptance portion approaches the wavelength of light. Accordingly, it is difficult to expect a CCD type or CMOS type image sensor to have better image quality or sensitivity than ever to thereby solve the abovementioned problems.
Under such circumstances, the structure of a solid-state imaging device, for example, described in JP-A-58-103165 has been reviewed. The solid-state imaging device has a structure in which a photosensitive layer for detecting red light, a photosensitive layer for detecting green light and a photosensitive layer for detecting blue light are stacked on a semiconductor substrate having signal readout circuits formed in its surface, by a film-forming technique and in which these photosensitive layers are provided as photo acceptance portions so that photoelectric conversion signals obtained by the photosensitive layers can be taken out to the outside by the signal readout circuits. That is, the solid-state imaging device has a photoelectric conversion film-stacked type structure.
According to the structure, limitation on design of the signal readout circuits can be reduced greatly because it is unnecessary to provide any photo acceptance portion on the surface of the semiconductor substrate. Moreover, sensitivity can be improved because efficiency in utilization of incident light is improved. In addition, resolution can be improved because light with the three primary colors of red, green and blue can be detected from one pixel (one photo acceptance portion). The problem of false colors can be eliminated. The problems inherent to the CCD type or CMOS type solid-state imaging device according to the related art can be solved.
Therefore, photoelectric conversion film-stacked type solid-state imaging devices described in JP-A-2002-83946, JP-T-2002-502120, JP-T-2003-502841 and JP-B-3405099 have been proposed in recent years. An organic semiconductor or nano particles may be used as the material of each photosensitive layer.
In the solid-state imaging device in which the photoelectric conversion films are stacked on the semiconductor substrate, signals of the three colors of red (R), green (G) and blue (B) can be detected simultaneously from the same pixel (the same photo acceptance portion) because the photoelectric conversion films are stacked as three layers, that is, a photoelectric conversion film for detecting red (R), a photoelectric conversion film for detecting green (G), and a photoelectric conversion film for detecting blue (B) are stacked.
FIG. 11 is a circuit configuration diagram of a signal readout circuit provided in a CMOS type image sensor according to the related art. In FIG. 11, a charge readout transistor 14 is connected to a photodiode 10 for detecting a photoelectric conversion signal of one color, an output transistor 11 and a reset transistor 13 are connected to the charge readout transistor 14, and a row selection transistor 12 is connected to the output transistor 11. That is, four MOS transistors are used. Incidentally, the reference numeral 20 designates a column signal line (image signal line); 21 designates a row selection signal line; 22 designates a reset signal line; 23 designates a DC power supply line; and 24 designates a charge readout signal line.
In the photoelectric conversion film-stacked type solid-state imaging device of the type in which the three colors are detected from one pixel, three colors each pixel can be detected simultaneously. Accordingly, when three signal readout circuits each the same as that shown in FIG. 11, that is, twelve MOS transistors in total are provided, signals of three colors can be read out simultaneously and in parallel.
In the photoelectric conversion film-stacked type solid-state imaging device, because it is unnecessary to provide any photo acceptance portion (photodiode shown in FIG. 11) in the semiconductor substrate, there is room for forming a large number of transistors in the semiconductor substrate. In order to make each pixel finer in the photoelectric conversion film-stacked type solid-type image sensing device, it is, however, preferable that the number of transistors in each signal readout circuit is as small as possible though there is still room in the semiconductor substrate.
In the case of a solid-state imaging device in which analog image signals read out by the signal readout circuits are converted into digital signals before image signals are output to the outside, it is necessary to provide an analog-to-digital conversion portion in an image signal output portion. In order to output image signals of three colors as digital signals simultaneously and in parallel, it is necessary to make the circuit configuration of the analog-to-digital conversion portion fine and manufacture three analog-to-digital conversion portions. Thus, there is a problem that cost will increase.
In recent years, there has been an increasing demand for a solid-state imaging device of the type in which a signal of another color, for example, an intermediate color (e.g. emerald color) between green (G) and blue (B) besides red (R), green (G) and blue (B) can be detected. In this case, it can be achieved when a photoelectric conversion film for detecting the emerald color is provided additionally. The required number of transistors in a signal readout circuit provided in the semiconductor substrate side is sixteen per pixel in total, so that four analog-to-digital conversion portions are required. For this reason, manufacturing cost increases. Moreover, a pitch between wirings for the image signal output portion to accept a signal from each pixel becomes narrow to thereby cause another problem that manufacturing becomes difficult.